LeetCode – Combination Sum III (Java)

Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.

Ensure that numbers within the set are sorted in ascending order.

Example 1: Input: k = 3, n = 7 Output: [[1,2,4]]
Example 2: Input: k = 3, n = 9 Output: [[1,2,6], [1,3,5], [2,3,4]]

Analysis

Related problems: Combination Sum, Combination Sum II.

Java Solution

public List<List<Integer>> combinationSum3(int k, int n) {
    List<List<Integer>> result = new ArrayList<List<Integer>>();
    List<Integer> curr = new ArrayList<Integer>();
    helper(result, curr, k, 1, n);
    return result;
}
 
public void helper(List<List<Integer>> result, List<Integer> curr, int k, int start, int sum){
    if(sum<0){
        return;
    }
 
    if(sum==0 && curr.size()==k){
        result.add(new ArrayList<Integer>(curr));
        return;
    }
 
    for(int i=start; i<=9; i++){
        curr.add(i);
        helper(result, curr, k, i+1, sum-i);
        curr.remove(curr.size()-1);
    }
}

Note the following relation in Java:

Integer is a subclass of Number and ArrayList is a subclass of List. But ArrayList is not a subclass of ArrayList.

java-subclass2

java-subclass1

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  • Suzy

    Thank you for the great solution!

  • Nilesh Barge

    What is time complexity of this solution?